The objective of this project is to understand with greater clarity the role of calcium ion (Ca2 ion) in neuromuscular transmission. The end-plate potential is abolished in the absence of extracellular Ca2 ion, while the miniature end-plate potentials (min.e.p.p.'s) persist under the same conditions and are accelerated by non-depolarizing agents. There are at least three possible explanations: 1) there are two separate transmitter release mechanisms in the nerve terminal. One requiring Ca2 ion, the other not; 2) there is a single mechanism with Ca2 ion a required cofactor only for release associated with depolarization; 3) there is a single, Ca2 ion-dependent mechanism and a large, inaccessible store of Ca2 ion in the nerve terminal activatable by min.e.p.p. accelerating agents. In order to distinguish among these possibilities, I will use standard electrophysiological and electron microscopic techniques to screen hydrated metal ions and determine what common properties among them the Ca2 ion-binding site is prepared to accept. The effects of Ca2 ion and Mg 2 ions inside the terminal will be examined with the aid of the toxic antibiotic and Ca 2/Mg 2 ions ionophore, A23187. The presence of a large, inaccessible store of Ca2 ion inside the terminal will be sought with the aid of heat, light, and mitochodrial uncouplers. Fractions of the potent venom of the spider, Phoneutria nigriventer, will be examined so that the component active at the neuromuscular junction may be characterized and correlated with investigations outlined above. The results of this investigation will significantly increase our understanding of the structure and function of the neuromuscular junction and increase its usefulness as a model synapse.